The clinical manifestations and biochemical changes in man resulting from exposure to 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) were studied and compared with the findings in patients with idiopathic Parkinson's disease. A unique pattern of change in the concentrations of monoamine metabolites in the cerebrospinal fluid was described. The neuropathological and neurochemical effects of MPTP in the rhesus monkey and beagle dog were studied. The toxic effects of MPTP are selective and result in cell loss of the A9 subgroup of dopaminergic cells in both species indicating that a biochemical difference exists between the A9 and A10 groups.